The present invention relates to induced CVD deposition of materials on masks and semiconductor chips, and particularly to ion beam induced deposition. A number of particular processes are known for depositing metal lines on a substrate by scanning a focused ion beam (FIB) along the locus of a desired growth line in the presence of a gas containing the metal to be deposited, so that the metal is selectively deposited in the regions scanned by the beam.
Unlike thermally induced or laser assisted CVD, the FIB induced deposition is a non-equilibrium process in which a great amount of energy is applied to a small area over a short time. Typically the deposited material contains a substantial quantity of impurities (over 25%) which include the ion beam species and various decomposition products of the metal precursor compound. Moreover, the deposition process competes with sputter-erosion of the material deposited by the ion beam. The rate of sputter-erosion thus places an upper limited on the ion beam energy effective to deposit metal. The metal precursor compound is generally fed locally to the region of the substrate, so that its vapor pressure does not impair the vacuum in the chamber along the ion beam path. The deposition process involves the formation of an adlayer of metal precursor on the substrate, which is broken down in situ to form the metal deposit. This adlayer, which may be a monomolecular or thicker layer, is reconstituted in between scans of the FIB, so that the adlayer formation rate poses a further constraint on the mechanics of deposition.
In addition to direct writing in this fashion, there are reports of using an FIB to form nucleation sites on the substrate, followed by thermally induced CVD to deposit a metal at the nucleated sites, as described in United States patent application Ser. No. 197,734 of Kubena et al, filed 23 May 1988. This pre-nucleation process reportedly operates at lower ion beam fluences than direct FIB induced CVD, and reportedly achieves greater purity of the deposited metal lines.
Among the metal precursor materials for FIB induced processes which have been reported are dimethyl gold hexafluoroacetylacetonate, iron pentacarbonyl, triisobutyl aluminum, and the hexacarbonyls of chromium, molbdenum and tungsten. Other precursors used to deposit impure but opaque lines for photomask repair include tungsten hexafluoride and tantalum pentaethoxide.
In general, these processes have deposited somewhat impure metal lines, of a resistivity substantially higher than the corresponding pure metals. Because of the relatively small path lengths involved, these deposits have nonetheless been suitable in some cases for forming conductive wiring in ICs. Further, the photon opacity of these deposits has made them suitable for photomask repair. However, the typical deposit thickness of 200-600 Angstroms and the porosity due to their organic impurities make these deposits relatively transparent to x-rays. There are prior suggestions of applying this technology to x-ray mask repair, by using as the ion beam a beam of gold or lead ions to provide a supplementary x-ray absorbing component in the deposition areas.
Overall, it may be said that while some FIB-induced CVD metal deposition processes have demonstrated feasibility if not commercial viability, the metal precursors and the deposition characteristics are by no means well understood or subject to predictable control. It is therefore desirable to determine other suitable precursors and detailed methods of pattern deposition and control.